U.S. patent number 4,070,206 [Application Number 05/688,476] was granted by the patent office on 1978-01-24 for polycrystalline or amorphous semiconductor photovoltaic device having improved collection efficiency.
This patent grant is currently assigned to RCA Corporation. Invention is credited to Robert Vincent D'Aiello, Henry Kressel, Paul Harvey Robinson.
United States Patent |
4,070,206 |
Kressel , et al. |
January 24, 1978 |
Polycrystalline or amorphous semiconductor photovoltaic device
having improved collection efficiency
Abstract
A body of semiconductor material having a first surface and a
second surface spaced from the first surface includes a first layer
along the first surface, a second layer along the second surface, a
third layer between and contiguous to the first and second layers.
The third layer is of a conductivity type opposite that of the
first and second layers so as to form first and second P-N
junctions respectively therebetween. The thickness of the third
layer is at least twice the minority carrier diffusion length of
the semiconductor material, so that carriers generated within the
third layer have a high probability of being collected by one of
the P-N junctions. The body includes means for electrically
connecting the first and second P-N junctions and means for
transferring the carriers collected at the first P-N junction to a
portion of the first surface.
Inventors: |
Kressel; Henry (Elizabeth,
NJ), D'Aiello; Robert Vincent (East Brunswick, NJ),
Robinson; Paul Harvey (Lawrenceville, NJ) |
Assignee: |
RCA Corporation (New York,
NY)
|
Family
ID: |
24764577 |
Appl.
No.: |
05/688,476 |
Filed: |
May 20, 1976 |
Current U.S.
Class: |
136/255; 136/249;
136/256; 136/258; 136/259; 257/53; 438/65; 438/96; 438/97 |
Current CPC
Class: |
H01L
31/0687 (20130101); H01L 31/056 (20141201); H01L
31/0547 (20141201); Y02E 10/52 (20130101); Y02E
10/544 (20130101) |
Current International
Class: |
H01L
31/068 (20060101); H01L 31/06 (20060101); H01L
31/052 (20060101); H01L 031/06 () |
Field of
Search: |
;136/89SJ,89CC,89PC,89TF
;357/30 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mack; John H.
Assistant Examiner: Weisstuch; Aaron
Attorney, Agent or Firm: Christoffersen; H. Morris; B. E.
Calder; D. N.
Government Interests
This invention herein described was made in the performance of work
under a NASA contract and is subject to the provisions of section
305 of the National Aeronautics and Space Act of 1958, Public Law
85-568 (72 Stat. 435; 42 U.S.C. 2457).
Claims
We claim:
1. A photovoltaic device comprising:
a body of polycrystalline or amorphous semiconductor material
capable of generating carriers by the absorption of solar radiation
having first and second opposed surfaces and including therein a
first layer of one conductivity type along the first surface, a
second layer of the same conductivity type as said first layer
along the second surface and a third layer between and contiguous
to both first and second layers, said third layer being of a
conductivity type opposite the conductivity type of said first and
second layers so as to form a first P-N junction between said first
and third layers and a second P-N junction between said second and
third layers, said third layer being of a thickness, from said
first P-N junction to said second P-N junction, of at least about
twice the minority carrier diffusion length of the semiconductor
material of said body;
a first pocket region at said first surface and extending as far as
the first P-N junction, said first pocket region being of the same
conductivity type as said third layer but of a higher
concentration;
means for electrically connecting the first and second P-N
junctions; and
means for both electrically contacting said second layer at said
second surface and reflecting solar radiation unabsorbed by said
body back into said body.
2. The photovoltaic device in accordance with claim 1 wherein said
electrically connecting means is a second pocket region at said
first surface crossing the first P-N junction and extending to the
second P-N junction, said second pocket region being of the same
conductivity type as the first and second layers but of a higher
concentration.
3. The photovoltaic device in accordance with claim 2 further
comprising a first electrode on said first pocket region at the
first surface, having an ohmic contact with said pocket region.
4. The photovoltaic device in accordance with claim 3 further
comprising a second electrode along the second surface having an
ohmic contact with said second layer.
5. The photovoltaic device in accordance with claim 4 wherein the
thickness of said third layer is about twice the minority carrier
diffusion length of the semiconductor material of said body.
6. The photovoltaic device in accordance with claim 2 further
comprising:
a first electrode on said first pocket region at the first surface,
having an ohmic contact with said first pocket region; and
a grid electrode on a portion of the first surface spaced between
said first and second pocket regions at the first surface, having
an ohmic contact with said first layer.
7. The photovoltaic device in accordance with claim 6 wherein said
grid electrode is on 5% or less of the surface area of the first
surface between said first and second pocket regions at the first
surface.
8. The photovoltaic device in accordance with claim 7 wherein said
grid electrode is finger shaped.
9. The photovoltaic device in accordance with claim 6 further
comprising solar radiation reflecting means on the second
surface.
10. The photovoltaic device in accordance with claim 9 wherein the
thickness of said second layer is on the order of at least twice
the minority carrier diffusion length of the semiconductor material
of said body.
11. The photovoltaic device in accordance with claim 6 further
comprising solar reflecting means spaced from said second surface.
Description
BACKGROUND OF THE INVENTION
The present invention relates to photovoltaic devices and more
particularly to photovoltaic devices having improved carrier
collection efficiency.
It is well known to those in the photovoltaic art that radiation at
the higher wavelength portion of the solar spectrum must travel
farther through a body of semiconductor material in order to be
absorbed. The farther into the semiconductor body radiation must
travel in order to be absorbed, the longer the carrier diffusion
length should be to assure collection of the generated carriers at
a P-N junction. Semiconductor materials which are of poor
crystalline quality, e.g., polycrystalline and amorphous
semiconductor materials, have diffusion lengths of generated
carriers which are relatively short. Thus photovoltaic devices of a
thickness sufficient to absorb the high wavelength portion of the
solar spectrum could not be fabricated utilizing these
semiconductor materials. These semiconductor materials are cheaper
to fabricate than the higher quality semiconductor materials, e.g.,
single crystalline silicon. Thus, it would be most desirable to
have a photovoltaic device, and especially a solar cell, which had
an improved collection efficiency so that the poor quality
semiconductor materials can be used more practically. Also, such a
photovoltaic device would improve the effectiveness of higher
quality semiconductor materials.
SUMMARY OF THE INVENTION
A photovoltaic device includes a body of semiconductor material
capable of generating carriers by the absorption of solar
radiation. The body has a first surface and a second surface
opposite the first surface. The body includes a first layer of one
conductivity type along the first surface, a second layer of the
same conductivity type as the first layer along the second surface,
and a third layer between and contiguous to both first and second
layers. The third layer is of a conductivity type opposite the
conductivity type of the first and second layers. There is a first
P-N junction between the first and third layers and a second P-N
junction between the second and third layers. The third layer has a
thickness, from the first P-N junction to the second P-N junction,
at least twice the minority carrier diffusion length of the
semiconductor material of the body. The body also includes a means
for transferring carriers collected at the first P-N junction to a
portion of the first surface and a means for electrically
connecting the first and second P-N junctions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the first embodiment of the
photovoltaic device of the present invention.
FIG. 2 is a schematic diagram of the equivalent electrical circuit
of the photovoltaic device of FIG. 1.
FIG. 3 is a perspective view of the first embodiment of the present
invention, as shown in FIG. 1, with means for both electrically
contacting the device and reflecting unabsorbed solar radiation
back into the device.
FIG. 4 is a perspective view of the second embodiment of the
photovoltaic device of the present invention.
FIG. 5 is a schematic diagram of the equivalent electrical circuit
of the photovoltaic device of FIG. 4.
FIG. 6 is a perspective view of the second embodiment of the
photovoltaic device of the present invention, as shown in FIG. 4,
with a radiation reflecting means on a surface of the device.
FIG. 7 is a perspective view of the second embodiment of the
photovoltaic device of the present invention, as shown in FIG. 4,
with a radiation reflecting means spaced from the surface of the
device.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, the photovoltaic device of the present
invention is designated as 10. For the purpose of describing the
photovoltaic device 10 of the present invention, the device 10 will
be described as a solar cell. The solar cell 10 includes a body 12
of semiconductor material capable of generating carriers by the
absorption of solar radiation. The semiconductor material of the
body 12 may be of a single crystalline semiconductor material, such
as silicon, or it may be of a polycrystalline or amorphous
semiconductor material, such as polycrystalline and amorphous
silicon. The body 12 has two substantially flat and opposed
surfaces consisting of a first surface 14, on which solar radiation
is capable of impinging solar cell 10, and a second surface 16.
Along the first surface 14 is a first layer 18 of the body 12, and
along the second surface 16 but spaced from the first layer 18 is a
second layer 20. Both first and second layers 18 and 20 are of the
same conductivity type. A third layer 22 is between and contiguous
to both the first layer 18 and the second layer 20. The third layer
22 is of a conductivity type opposite that of the first and second
layers 18 and 20. Thus, there is a first P-N junction 24 between
the first layer 18 and third layer 22, and a second P-N junction 26
between the second layer 20 and third layer 22. For the purpose of
explaining the solar cell 10 of the present invention, the first
and second layers 18 and 20 are assumed to be of P type
conductivity while the third layer 22 is assumed to be of N type
conductivity, although the conductivities of the three layers, 18,
20 and 22 can be opposite that described.
The third region 22 between the first P-N junction 24 and second
P-N junction 26 is of a thickness of at least two times the
minority carrier diffusion length of the semiconductor material of
the body 12. While it is preferable that the thickness of the third
region 22 be two times the minority carrier diffusion length, the
thickness of the third region may be larger than two times the
minority carrier diffusion length, for reasons subsequently
explained.
The body 12 includes means for transferring carriers which are
collected at the first P-N junction 24 to a portion of the first
surface 14. The carrier transfer means can be a first pocket region
28 which is at a portion of the first surface 14 and extends into
the body 12 at least to the first P-N junction 24. Spaced from the
first pocket region 28 is means for electrically connecting the
first and second P-N junctions 24 and 26. The connecting means can
be a second pocket region 30 which is at a portion of the first
surface 14 and extends into the body 12 crossing the first P-N
junction 24 and continuing to at least the second P-N junction 26.
The first pocket region 28 is of the same conductivity type as the
third layer 22 but is of a higher dopant concentration. The second
pocket region 30 is of the same conductivity type as the first and
second layers 18 and 20 but of a higher dopant concentration.
A first electrode 32 is in contact with the first pocket region 28
at the first surface 14. On the second surface 16 and in contact
with the second layer 20 is a second electrode 34. Both first and
second electrodes 32 and 34 form ohmic contacts with the
semiconductor body 12. The first and second electrodes may be of a
single layer of a metal, such as aluminum, or they may be of
multilayers, such as a first layer of chromium directly on the body
12 and a second layer of gold on the chromium layer.
In the operation of the solar cell device 10, solar radiation 40
first encounters the cell 10 at the first surface 14. Solar
radiation penetrating into the body 12 will be absorbed by the body
12 resulting in the generation of electron-hole pairs. The lower
wavelength portion of the solar spectrum contains photons of higher
energy than the higher wavelength portion of the solar spectrum.
Consequently, the lower wavelength radiation is absorbed close to
the first surface 14 in the first region 18. The higher wavelength
portion of the solar spectrum must travel deeper into the body 12
before it will be absorbed, typically in the third and second
layers 22 and 20.
As stated previously, the third layer 22 is preferably of a
thickness of about twice the minority carrier diffusion length of
the semiconductor material of body 12. As a consequence of the
layer 22 being of such a thickness more radiation will be absorbed
in the third layer 22 then if it were only of a thickness of a
minority carrier diffusion length. The carriers generated within
the third layer 22 have a very high probability of being collected
since they are generated at a distance no farther than a minority
carrier diffusion length from either the first or second P-N
junctions 24 and 26. Therefore, the cell 10 has an increased
carrier collection efficiency.
The thickness of the third layer 22 can be larger than twice the
minority carrier diffusion length resulting in the absorption of
more radiation, but the probability of collecting carriers which
are generated farther than a minority carrier diffusion length from
either P-N junctions 24 or 26 begins to be reduced. In addition,
the increased thickness of the third layer 22 will add to the
series resistance of the cell 10. Nevertheless, a trade-off between
increasing layer 22's thickness beyond twice the minority carrier
diffusion length and reducing collection efficiency may be made by
one skilled in the art to optimize cell efficiency. Factors which
one may consider in such an optimization include what semiconductor
material is being used and what portion of the solar spectrum is to
be utilized to its fullest extent. Since the solar cell 10 of the
present invention provides increased carrier collection efficiency
for a third layer 22 which is relatively thick, i.e., at least
twice the minority diffusion length, the cell 10 may be of
semiconductor materials which have shorter diffusion lengths than
single crystalline materials. Semiconductor materials having a low
diffusion length are, for example, polycrystalline and amorphous
crystalline semiconductor materials.
The function of the first pocket region 28 is to transfer the
carriers collected at the second P-N junction 26 to the first
surface 14 and first electrode 32, while the function of the second
pocket region 30 is to electrically connect the first and second
P-N junctions 24 and 26. Referring to FIG. 2, the solar cell device
10 is in essence electrically equivalent to two solar cells
connected in parallel with each other. In FIG. 2 the first and
second P-N junctions 24 and 26 are represented by the electrical
symbol for a diode while the third region 22, common to both solar
cells, is represented by the resistor A and the second pocket
region 30 is represented by the resistor B. The cell 10 makes
electrical contact to external circuitry at the first electrode 32,
represented by point C, and second electrode 34, represented by
point E.
In the fabrication of the cell 10, vapor or liquid phase epitaxy
processes well known in the art may be utilized to sequentially
grow the third layer 22 and first layer 18 on a substrate which
will become second layer 20. If fabricated by vapor epitaxy
methods, where the semiconductor material to be grown is single
crystalline silicon, a P type conductivity silicon substrate is
placed in the vapor phase deposition chamber. A source gas such as
silane or silicon tetrachloride or dichlorosilane is bled into the
vapor deposition chamber along with appropriate doping gases to
first form the third layer 22 on the substrate and then the first
layer 18 on the third layer 22. Typically, the first layer 18 will
have a P type doping concentration on the order of or greater than
10.sup.18 atoms/cm.sup.3 and be of a thickness less than or equal
to about 1 micron. The third layer 22 has an N type doping
concentration on the order of 10.sup.14 to 10.sup.17
atoms/cm.sup.3. As previously stated the thickness of the third
region 22 is typically on the order of twice the minority carrier
diffusion length of the semiconductor material. Thus depending on
whether the semiconductor material is single crystalline or
polycrystalline or amorphous, the thickness of the third layer 22
can range from as high as the order of 100 microns to as low as an
order of 5 microns in thickness.
After the third and first layers 22 and 18 have been formed, the P
type substrate, which may be as thick as 10 mils, is either lapped
or ground to a thickness on the order of a couple of mils, thereby
forming the second layer 20. If the second layer 20 were more than
a couple of mils thick it would add to the series resistance of the
cell 10.
Alternatively, the second, third and first layers 20, 22 and 18 can
be formed by sequentially depositing the layers on a heavily doped
N.sup.+ substrate of silicon. Then by using a preferential etch the
N.sup.+ substrate is removed.
After forming the second, third and first layers 20, 22 and 18, an
oxide layer, such as silicon dioxide, is deposited by well known
evaporation techniques or grown by oxidation techniques on the
first surface 14. By photolithographic and etching techniques a gap
is formed into the oxide layer at the first surface 14 where the
second pocket region 30 is to be formed. The body 12 with the
patterned oxide layer is then placed in a diffusion furnace and a P
type dopant, such as boron or aluminum, is diffused into the body
12, crossing the first P-N junction 24 and continuing to at least
the second P-N junction 26, thereby forming second pocket region
30. Typically, the conductivity concentration of the second pocket
region will be on the order of 10.sup.20 atoms/cm.sup.3. The
patterned oxide layer is then removed by standard stripping
techniques such as by etching with buffered HF. Then another
patterned oxide mask is formed like the first patterned oxide mask
on the first surface 14, but with a gap in the oxide layer where
the first pocket region 28 is to be formed. Again the wafer is
placed into a diffusion furnace; this time an N type dopant, such
as phosphorous or arsenic, is diffused into the body 12 to at least
the first P-N junction 24, thereby forming the first pocket region
28. Typically the first pocket region 28 will be of a dopant
concentration on the order of 10.sup.20 atoms/cm.sup.3. It is
preferable to first form the second pocket region 30 before forming
the first pocket region 28. As is well known in the art, dopants
diffuse less rapidly with time, thus, while the first pocket region
28 is being formed the second pocket region 30 which has already
been formed will diffuse little more.
Next, the body 12 is placed in an evaporation furnace, where by
standard evaporation techniques well known in the art, the second
electrode 34 of a metal, such as aluminum, is deposited on the
second surface 16. A metallic layer is then deposited on the first
surface 14 and by well known photolithographic and etching
techniques the pattern of the first electrode 32 is defined on the
metal layer and the unwanted portion of the metal layer is etched
away forming first electrode 32, and thereby completing the
fabrication of the photovoltaic device 10.
Referring to FIG. 3, it is anticipated by the present invention
that means for both electrically contacting second layer 20 and
reflecting unabsorbed solar radiation back into the body 12 may be
on the second surface 16. The means will increase the radiation
absorption of body 12. Means for both electrically contacting and
radiation reflecting are well known in the art. By way of example,
the means may include a non-continuous oxide layer 52 having
openings 54 therethrough to the second surface 16. The openings 54
can be in any form well known to those in the art such as grid
pattern. In the openings 54 and on the oxide layer 52 is a layer 56
of a material which is reflective to solar radiation, such as
metal.
Referring to FIG. 4, a second embodiment of the photovoltaic device
of the present invention is designated as 110. For the purpose of
explaining the photovoltaic device 110 it will be described as a
solar cell. The solar cell 110 is very similar in structure to that
of the solar cell 10 of the first embodiment except that the solar
cell 110 does not have a second electrode 34 on the second surface
16. Instead solar cell device 110 has a grid electrode 136 on the
first surface 1/4 between the first and second pocket regions 128
and 130 at the first surface 114. The grid electrode 136 can be in
any grid shape well known to those in the solar cell art such as
the finger type shown in FIG. 4, but it should only cover a small
portion of the first surface 114, i.e., cover less than about 5% of
the area of the first surface 114. If the electrode 136 is only on
a small portion of first surface 114 it can prevent only a small
portion of the solar radiation from impinging on first surface 114.
The grid electrode 136 can be of a single layer of a metal, such as
aluminum or it can be of multi-layer, such as a first layer of
chromium directly on the first surface 114 with a layer of gold on
the chromium layer.
The operation of the solar cell 110 is similar to that of the solar
cell 10 of the first embodiment. Like the first embodiment, the
cell 110 is electrically equivalent to two solar cells in parallel
as shown in FIG. 5. The only difference between the schematics
shown in FIG. 5 for the second embodiment and that shown in FIG. 2
for the first embodiment is that electrical contact to external
circuitry is made at the first electrode 132, represented by point
C, and at the grid electrode 136, represented by point D, for the
second embodiment.
Because the contacts are only on a portion of one surface of the
solar cell 110 the second embodiment of the present invention has
an advantage over the first embodiment in that solar radiation can
impinge either surface. Also, now that solar radiation can impinge
either surface, a reflector can be at the second surface 116
without the interference of a contact on the second surface 116.
This advantage of the second embodiment can be used in several
applications. In a first application, as shown in FIG. 4, solar
radiation 140 can directly impinge the second surface 116 without
any interference from the electrodes.
In a second application, referring to FIG. 6, a means for
reflecting radiation 150 is on the second surface 116 so that solar
radiation which initially impinged the first surface 114 and passed
through the body 112 may be reflected back into the body 112 by the
reflector 150 for further absorption. The radiation reflecting
means 150 can be an oxide layer 152 directly on the second surface
116 with a layer 154 which is reflective to solar radiation on the
oxide layer 152. The oxide layer may be, for example, silicon
dioxide and the layer 154 may be a metal. The reflecting means 150
shown in FIG. 6 is only by way of an example, since other
reflecting means well known to those in the art could also be
utilized.
Thirdly, a radiation reflecting means 160 such as the parabolic
mirror in FIG. 7, is spaced from the second surfce 116 so that both
filtered solar radiation 156, i.e., solar radiation which has
traveled through the body 112 after first impinging the first
surface 114, and unfiltered solar radiation 158, i.e., solar
radiation which has never entered the body 112, may be directed
toward the second surface 116. The parabolic mirror is only shown
as an example since other spaced reflecting means 160 well known to
those in the art can also be utilized.
In the instances where unfiltered solar radiation first impinges
the second surface 116 the thickness of the second layer 120 should
typically be less than a minority carrier diffusion length of the
semiconductor material of the body 112. The reason for this
requirement in thickness is that the unfiltered solar radiation
consists of high energy photons, i.e., the lower wavelength portion
of the solar spectrum, which are absorbed quite readily by the
semiconductor material within a short distance. Therefore to assure
that these carriers will be able to diffuse to the second P-N
junction 126, it is preferable that the thickness of the second
layer 120 be less than a minority carrier diffusion length. In
contrast, when the only solar radiation striking the second surface
116 is solar radiation which has been filtered through the body 112
and is just being reflected back into the body 112, the thickness
of the second layer 120, in these circumstances, should be on the
order of at least twice the minority carrier diffusion length. The
reason for the first layer 118 being thicker in this instance is
because the filtered solar radiation contains photons of low energy
levels which will have to travel farther through the semiconductor
material before they are absorbed.
The fabrication of the second embodiment of the present invention
is substantially the same as that of the first embodiment. The
difference in fabrication is that both first electrode 132 and grid
electrode 136 can be formed on the incident surface 114 at the same
time. In forming the first electrode 132 and grid electrode 136 a
metallic layer is first deposited on the first surface 114. Then by
photolithographic techniques well known in the art the pattern of
the electrode 132 and the electrode 136 are defined on the metallic
layer and then by etching techniques well known in the art the
unwanted portion of the metallic layer is removed thereby forming
the first electrode 132 and grid electrode 136.
The first and second embodiments of the photovoltaic device of the
present invention provides higher collection efficiency of carriers
formed in the semiconductor body of the device. This makes possible
the utilization in photovoltaic devices semiconductor materials
having a short diffusion length and improves the carrier collection
efficiency of photovoltaic devices using semiconductor materials of
longer diffusion lengths.
* * * * *